1. Field of the Invention
The technical field of the present invention relates to a process for removal of salts thermally non-regenerable in a temperature range from 0° C. to 200° C. from organic solvents, such as aqueous amine solutions, by evaporation. The inventive process is to support a process for the removal of acid gas constituents, such as CO2 and H2S, and reprocess a partial stream from the solvent main stream containing the thermally non-regenerable salts. The thermally non-regenerable salts are formed, for example, by the feed of traces of strongly acidic gases, such as Cl2, HCOOH, SO2, SO3, NO2, etc. together with the input gas supplied to the absorber of the scrubber.
2. Description of the Related Art
Regeneration of a chemical scrubbing medium is achieved by the fact that constituents such as acid gas constituents, e.g. CO2, H2S, HCN, mercaptans, etc. chemically bound in an absorption are thermally reconverted at elevated temperatures into their initial constituents without this causing an alteration of the chemical scrubbing medium in its chemical structure. Thus, it represents a reversible chemical process that can be influenced by a change in temperature.
This principle can be applied in particular to light-volatile, relatively weakly acidic constituents as outlined hereinabove by way of examples. Aqueous amine solutions, e.g. MEA, DEA, TEA, DGA, DIPA, MDEA, or solvents utilized in the Flexsorb process, which is known to an expert, are very often applied as scrubbing media/solvents for such constituents. Thus, thermally regenerable salts are formed, the scrubbing medium/solvent being recoverable in a temperature range from 0° C. to 200° C., and the absorbed gases in the regeneration column being again removed from the solution.
Amine constituents are also utilized as additives in what are called hybrid scrubbing media/solvents, which an expert in general understands to be a combination of physical and chemical scrubbing media, e.g. sulfinol.
If only light-volatile, weakly acidic constituents are available in a given gas stream, one of these afore-mentioned scrubbing media/solvents can be utilized very advantageously for the removal of these acid gas constituents. Such a process is, for example, described in DE 3408851 A1. Here, the removal of CO2 and/or H2S from gases by means of an aqueous absorption liquid containing methyl diethanol amine is described. Accordingly, the burdened absorption liquid is expanded in at least two expansion stages for regeneration. A gas containing CO2 and/or H2S is withdrawn at the heads, and the absorption liquid is enriched at the bottom and then passed into a second vessel. The bottom of the final expansion stage contains the regenerated absorption liquid.
In most applications, however, more strongly acidic constituents such as chlorides, SOX constituents, sulfates, thiosulfates, thiocyanates, organic acids, such as formiates, acetates, etc. entrained by the gas stream also get into the solvent. These more strongly acidic constituents cannot be removed from the solvent by implementing the thermal measures outlined hereinabove and described by way of example in DE 3408851 A1, so that these salt-like constituents termed heat stable salts, and hereinafter also termed HSS, are continually enriched. Hence, this concerns salts thermally non-regenerable in a temperature range from 0° C. to 200° C.
For two different reasons, this enrichment is extremely disadvantageous for the scrubbing process:
On one hand, due to the chemical bond of these constituents with the active scrubbing constituent, e.g. amine, the active scrubbing capacity for acid gas constituents is (at least) proportionally reduced. On the other hand, a rising concentration of HSS constituents leads to a strongly increasing corrosiveness of the scrubbing solution. As a rule, the HSS content for a still tolerable corrosiveness of the solution should not exceed 3% by wt.
On account of these reasons, entrained HSS constituents must be removed and/or sluiced out from the solvent. The simplest, though most expensive possibility is discharging such an amount of solvent that corresponds to the amount of HSS constituents carried in. This procedure is generally termed “repulsive mode”. In most cases, however, considerably more solvent gets lost than is sluiced out as HSS. For example, this occurs if the HSS concentration amounts to 3% by wt. and the amine concentration, for instance, amounts to 40% by wt.
According to prior art in technology, a host of different processes is available to selectively remove HSS constituents from the process, e.g. distillative processes and ion exchanger processes, too. Such a process treatment stage is also termed reclaimer or reclaimer unit. One of those processes operating in the most selective manner is the ion exchanger process. However, relatively large amounts of wastewaters are produced there due to scrubbing and flushing processes.
A semi-continuously working evaporator, too, is frequently applied. Such an evaporator operates by feeding a partial stream of the solvent burdened with HSS and withdrawn from the main solution circulation to an evaporator, the chemically reacting scrubbing medium constituent, e.g. MEA from an aqueous MEA solution, supplied to the evaporator being exactly evaporated in the same extent as it is supplied to the evaporator. Obtained here is an amine concentration of e.g. 80% by wt. MEA in the solution. This is the case because under these process conditions the MEA quantity existing in the input stream precisely exists in the gas phase.
Had there been no HSS constituents and no other high-boiling constituents in the solvent input stream, the process could be continued for an arbitrarily long period of time. On account of the HSS constituents existing in the input solution, however, an enrichment of these salt-like constituents occurs in the evaporator.
When a concentration of e.g. 10% by wt. of a salt freight in the evaporator is achieved, the process in the evaporator is reversed from a continuous mode to a discontinuous mode. By addition of a strong base, e.g. NaOH, the HSS constituents chemically bound to amine are bound to the Na+ ion so that the amine hitherto fixed to HSS can also be evaporated. By addition of water vapour and an external heating, the amine solution now existing in the evaporator is evaporated until almost no amine can be detected any more in the gas phase (e.g. <0.1% by vol.).
If this point is reached, the whole charge existing in the evaporator is drained from the evaporator and must be disposed of. However, there still exists a residual concentration of 20 to 30% by wt. of amine in the liquid phase.
If one compares the application of this reclaimer type with the simple repulsive mode outlined hereinabove, it results in a reduction of the amine loss by factor 3 to 4 as compared with the repulsive mode, which would correspond to a reclaimer efficiency degree of 66 to 75% relative to a mere repulsive mode. But the loss of expensive solvent (2 to 10  per kg) is still quite high. However, in terms of capital investment costs and consumption of utilities, the evaporator reclaimer described above is very cost-efficient as compared with an ion exchanger, even though two evaporators of this kind are generally implemented to ensure permanent reclaimer operation. This is the case, in particular, if the reclaimer steam evolving on evaporation is passed into the sump of an amine regenerator where it can be exploited as stripping steam for thermal stripping of amine from acid gas constituents, thus reducing the external amount of heat required for amine regeneration. Had there not been the residual losses of amine, the evaporator concept described above would be the ideal reclaimer concept for chemically reacting organic solvents in aqueous solution.